Metal strip after the strip-forming process, e.g. rolling, may have defects with respect to planarity of the strip which can be length differences with respect to longitudinal zones of the strip across the width thereof. The defects generally are detectable as planarity defects and it is customary to subject the strip to leveling depending upon the deviation of the strip from true planarity. Indeed, deviation from planarity may arise not only in strip-rolling lines but also in strip-processing lines and the lack of planarity can be a function of the deformation by rolling, of various leveling methods and annealing or heat treatment generally of the strip.
Especially in the cold rolling of strip, it can be observed that there is a certain tendency toward corrugation of the strip which can arise because of differences in strip thickness across the strip or longitudinally therealong and as a consequence of the different degrees of plastic deformation which can be carried out on the strip or can arise from the cold rolling process. Apart from deviations in planarity there may be distortion in the strip itself which are evident as strip camber or edge waviness.
As a consequence, it has been customary to carry out planarity measurements in metal strip traversing a strip-rolling line or a processing line. For example, the tension force in the strip can be determined and divided by the strip width and strip thickness and utilized to provide a feedback signal which is used to control strip tension. Corrugations in the strip have created problems where the strip was to be used in some industries, for example in the automobile sector, especially when the lack of planarity of the strip exceeded certain limits. As a measure of deviation from planarity, a so-called I-unit (IU) has been developed. An IU corresponds to a tension difference of 10−5 across a strip or across a longitudinal segment of the strip. For example, when the tension differences across the strip are measured in length differences, for example Δl/l is less than 10−5, the lack of planarity is deemed to be less than 1 IU. In this relationship Δl is, of course, the difference in length as measured cross the strip while l is the length over which that difference is applicable.
Any producer of strip must as a rule determine the planarity limits of the product and thus planarity measurements must be taken and these measurements utilized within the rolling line or strip processing line to control the rolling or strip processing line or to identify the quality of the strip produced. Where such measurements are taken, they can be used in real time to regulate a shaping or processing process, i.e. in parallel therewith.
EP 1 116 952 A2 describes a contactless method of measuring planarity while a system utilizing measurement rollers in contact with a continually traveling strip to determine the tension therein is described for example DE 199 18 699 A1.
While the contactless measurement process mainly utilizes acoustic waves, ultrasonic waves or electromagnetic measurements to detect the planarity, the roller approach provides a magnetic measurement system and has the greatest use in practice. The measurement roll approach of DE 199 18 699 A1 utilizes a multiplicity of measurement pickups which can be received in recesses and spaced from the wall of the measurement roller. The measurement roller can then be subdivided into so-called disk segments which have been described for example in EP 1 182 424 A1. With the aid of the sensors, radial force measurements are made where the strip is looped around a roller and the output signals are measurements of the local tension which can be divided by the width of the segment and the strip thickness.
The planarity defects resulting from length differences correspond to varying tensions at the respective segments since the latter are measurements of the extent of elongation in the metal strip and the effect thereof on the planarity. Reference may be to the work “Formabweichungen in Bändern: Einteilung, Entstehung, Messung und Beseitigung sowie quantitative Bewertungsmethoden” (Shape Deviations in Strip: Classification, Creation, Measurement and Evidence as well as Quantitative Evaluation Methods by Gert Mücke, Kai F. Karhausen and Paul-Dieter Pütz (Stahl and Eisen 122 (2002) No. 2, Pages 33 ff).
The known mechanical measurement methods have some basic drawbacks in that only a limited number of measurement points are obtained over the strip width with local tension measurement techniques. That means that a sufficiently precise resolution requires a large number of sensors and hence a comparatively costly measurement roller and thus significant expense in evaluating the results. Furthermore, measurements at edges of the strip are especially problematical because it is there possible that a sensor will only partially cover the strip edge or be looped by the strip. The result can be errors. Errors can also be produced when the position of the strip edge is measured.